How can I import multiple result.xml files when creating a new reports?
Just select them from your filebrowser and drag'n drop them to the dropzone on the upload form. You can also select multiple files by clicking the "Browse..." button and selecting the files ctrl+click or shift+click. Both dropzone and the "Browse..." require modern browser (e.g. Firefox 3.6+ or Chrome 8+) in order to work properly with multiple files.
How can I move a test report under different Release, Profile, Test Type or HW?
See moving a test report for the details.
Can I download CSV file from the QA Reports and upload it again in order to create a new test report?
Noup. CSV export downloaded from the QA Reports is meant for analysis, i.e., you can create custom reports and graphs with the data in Excel. For that reason, the export CSV is in a different format and contains more information than the import CSV. If you want to use the same CSV file to import multiple results, please keep the original file available at your workstation and modify it as necessary. See Test Result CSV format for the details.